German Patent Specification DE No. 3,143,121 which corresponds to U.S. Pat. No. 4,440,833, describes such a synthetic resin composition and a fibrous substrate material for printed circuit boards, in particular paper, in which a phenol-formaldehyde resin is impregnated which is mixed with a softening polyurethane component to allow for, for example, holes to be punched or pierced in the material without the substrate material becoming torn at the edges of the holes.
A disadvantage of such a material to which a softener is added, is the higher degree of sagging of the material, in particular at a higher temperature, for example, over a wavesoldering bath as used to attach electric and electronic components to the printed circuit board.